Supercapacitors are electrical energy storage systems that are particularly advantageous for applications that require electrical energy to be conveyed at high power. Their aptitude for rapid charging and decharging, their increased service life compared to a high-power battery make them promising candidates for many applications. Supercapacitors generally consist of the combination of two conductive porous electrodes having a high specific surface area, submerged in an ionic electrolyte and separated by an insulating membrane known as a “separator”, which enables ionic conductivity and prevents electrical contact between the electrodes. Each electrode is in contact with a metallic collector that enables the exchange of the electric current with an outside system. Under the influence of a potential difference applied between the two electrodes, the ions present within the electrolyte are attracted by the electrode surface having an opposite charge, thus forming an electrochemical double layer at the interface of each electrode. The electrical energy is thus stored electrostatically by charge separation. The expression of the capacitance C of a supercapacitor is identical to that of a conventional capacitor, namely:
C=ε·S/e, where ε denotes the permittivity of the medium, S the surface area occupied by the double layer, and e the thickness of the double layer.
The capacitances that can be achieved within supercapacitors are nevertheless much higher than those commonly achieved by conventional capacitors, due to the use of carbonaceous electrodes having a maximized specific surface area and the extreme thinness of the electrochemical double layer (typically a few nanometers thick). These carbonaceous electrodes must inevitably be conductive in order to ensure the transport of the electrical charges, porous in order to ensure the transport of the ionic charges and the formation of the electrical double layer over a large surface area, and chemically inert in order to prevent any unwanted energy-consuming reaction.
The energy E stored within a supercapacitor is defined according to the conventional expression for capacitors, namely:
E=½ CV2, where V is the potential of the supercapacitance.
The capacitance and the potential are therefore two essential parameters that it is necessary to optimize to promote energetic performances. The capacitance depends on the porous texture actually accessible by the electrolyte. However, for applications in transport and especially for an electric vehicle, it is necessary to have a high energy density in order to limit the onboard mass of the supercapacitor, which makes it necessary to have a high mass capacitance.
The potential of a supercapacitor mainly depends on the nature of the electrolyte used and especially on its electrochemical stability, it being specified that there are two major families of electrolytes which are organic and aqueous electrolytes.
Organic electrolytes are based on an organic salt (typically a quaternary ammonium salt) dispersed in an organic solvent. Although certain organic electrolytes make it possible to achieve an operating potential of 2.7 V, organic electrolytes have the drawback of being expensive, inflammable, toxic and potentially polluting, thus posing safety problems for use in a vehicle.
Aqueous electrolytes are on the contrary inexpensive and nonflammable, which renders them attractive for certain applications. The potential that can be applied with these electrolytes is limited to the water stability range, of the order of 1.2 V. Among the aqueous electrolytes that can be used in a supercapacitor, mention may, for example, be made of aqueous solutions of sulfuric acid, of potassium chloride, of potassium sulfate or of other salts in an acid, basic or neutral medium.
In order to be able to achieve operation with high powers, the resistance to the passage of the current in the system must be very low. Indeed, this resistance, which is the sum of the resistances of the various components of the system and especially the resistance of the electrolyte, and that of the current collectors, generates losses by the Joule effect which reduce the efficiency of the supercapacitor. An important contribution is the resistance of the interface between the metallic current collector and the carbonaceous active material of the electrode, which is dependent on the quality and on the nature of the contact. It is therefore necessary to use an electrode that has a good adhesion with the metals used for the collectors in order to improve the electrical contact.
There are currently several techniques for the preparation and implementation of electrodes based on carbon powder for supercapacitors operating with an aqueous ionic electrolyte.
A first technique for example described in documents US-A1-2007/0146967, U.S. Pat. No. B2-7,811,337, EP-A1-1 255 261, US-A1-2006/0000071, U.S. Pat. No. B2-7,316,864, US-A1-2011/0176255 and US-A1-2009/0110806, consists in dispersing the carbon powder with a hydrophobic binder in a solvent or organic monomer, in coating the composition onto a current collector then in evaporating this solvent or in polymerizing this monomer. A typical example of a hydrophobic binder used is poly(vinyl difluoride) (PVDF) used in N-methyl-2-pyrrolidinone (NMP). This technique has the drawback of using toxic products and of releasing volatile organic compounds (VOCs). Furthermore, since the binder cannot be swollen by the aqueous electrolyte, it is necessary to have a large macroporosity of the electrode in order to enable the diffusion of this electrolyte into the electrode, which mechanically weakens the electrode and does not make it possible to have a maximum capacitance.
Another technique consists in using a hydrophobic binder, such as a latex in water. This technique, for example described in documents U.S. Pat. No. B1-6,356,432, US-A1-2010/0304270 or JP-A 1-11-162794 makes it possible to avoid the release of organic solvent but does not enable an optimum exchange in diffusion of the electrolyte in the electrode, which does not lead to an optimum capacitance for the electrode.
Document US-A1-2009/0325069 presents another technique which consists in utilizing hydrophilic binder systems used in an aqueous medium, but the only electrolyte tested in this document in connection with the electrodes manufactured by means of this system is an organic electrolyte (see paragraph [0180]) consisting of ethylammonium tetrafluoroborate dissolved in propylene carbonate. The binder systems tested in this document are based on a propylene/ethylene or propylene/butene/ethylene copolymer modified by maleic anhydride, and additionally comprise N,N-dimethylethanolamine (DMEA) or triethylamine (TEA) as basic compound. These systems are prepared using an organic solvent consisting of n-propanol or tetrahydrofuran, and optionally comprise a reactive surfactant.
A major drawback of such a hydrophilic binder system lies in the dissolving thereof in the water of a specifically aqueous electrolyte.